A helium leak detector essentially comprises two portions as shown in FIG. 1 the accompanying drawing: (1) a pumping station including a sliding vane rotary vacuum pump 1, a pump isolating valve 2 and an air inlet valve 3, and finally an inlet 4 for connection to the chamber to be tested, said inlet 4 being likewise capable of being closed; and (2) a gas analyzer 5 including an inlet valve 6 and connected to the pumping station.
The analyzer 5 detects and measures the quantity of helium which enters the apparatus via the inlet e.
Tests are performed as follows:
The valves 3 and 6 are closed, the valve 2 is opened and the inlet 4 is connected to the chamber to be tested. The pump 1 is switched on to evacuate the chamber. The valve 2 is then closed. The inlet valve 6 is then opened and measurements are performed by directing a jet of helium against the walls of the chamber being tested. If the chamber has a leak, helium enters the chamber and thence passes via the inlet 4 to the analyzer 5 which detects and measures the presence of helium and thus the leak in the chamber under test.
However, it sometimes happens that the analyzer 5 detects a "leak" signal before the chamber under test has had helium applied thereto. This comes from pollution of the installation due, for example, to an earlier test during which helium had entered the circuit. During a subsequent test, the walls and the components of the apparatus degas thereby setting up background noise which appears to be a leak. The presently adopted solution consists in running the vane pump 1 for a certain length of time with the valve 3 closed and the valve 2 open. In come cases this length of time may be very long since the background noise level can be very high. For example, such pumping may continue for two hours. It used to be believed that this pollution is mainly due to degasing of the walls and of the oil contained in the sump of the vane pump, and that this degasing was therefore very slow.
However, the inventor has finally observed after lengthy research into the causes of this pollution, that the main cause stems from a concentration of helium in the free space situated in the sump of the vane pump above the surface of the oil.
If the inlet to a vane pump is designated by e and the space in the sump situated above the oil is designated by S, with said space being substantially at atmospheric pressure, the compression ratio k of a pump of this kind for helium is about 10.sup.7, i.e. the ratio of helium partial pressure is Ps/Pe.perspectiveto.10.sup.7.
If the helium contained in the space S is at the same concentration as the helium which is normally contained in the atmosphere, i.e. as a partial pressure Ps=5.10.sup.-6 atmospheres, then: EQU Pe=(5.times.10.sup.-6)/10.sup.7 =10.sup.-13
and a helium partial pressure of such a value in the inlet ducts of the analyzer is low enough to avoid producing background noise at a level which could cause difficulties. However, if a large amount of helium is injected into the apparatus, the space S contains a much higher proportion of helium, for example at a partial pressure of 10.sup.-1 atmospheres. This gives rise to a partial pressure at the inlet e and thus in the inlet ducts to the analyzer of: EQU Pe=10.sup.-1 /10.sup.7 =10.sup.-8 atmospheres.
A helium partial pressure at this level in the inlet limits the performance of the analyzer by creating background noise which is readily detectable by the analyzer 5 even when there is no leak.
The remedy is thus to reduce the helium partial pressure Ps in the sump of the vane pump.